<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    body, html{
        width: 100%;
        height: 100%;
    }

    header{
        width: 100%;
        height: 120px;
    }

    ul{
        display: flex;
        flex-flow: row nowrap;
        width: auto;
        height: 80px;
        list-style: none;
        overflow-x: scroll;
    }
    li{
        list-style: none;
        width: 80px;
        flex-shrink: 0;
        height: 100%;
        background: #ff9800;
        margin-left: 20px;
    }
</style>
<body>

<header>
    <ul>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
    </ul>
</header>

</body>
</html>